spi: omap3: fix set_wordlen() reading from incorrect address for CHCONF

spi: omap3: fix set_wordlen() reading from incorrect address for CHCONF

_omap3_spi_set_wordlen() indexed the regs->channel[] array with the
old wordlen (instead of the chipselect number) when reading the current
CHCONF register value. This meant it read from the wrong memory location,
modified that value, and then wrote it back to the correct CHCONF
register. The end result is that most slave configuration settings would
be lost, such as clock divisor, clock/chipselect polarities, etc.

Fixes: 77b8d04854f4 ("spi: omap3: Convert to driver model")
Signed-off-by: David Rivshin <drivshin@allworx.com>